1. Field of the Invention
This invention relates to the acceleration of solar evaporation of water and more specifically to the acceleration of solar evaporation of water from trona-process waste pond liquors by use of certain soluble inert dyes.
2. DESCRIPTION OF THE PRIOR ART
Naturally occuring trona, consisting mainly of sodium sesquicarbonate (Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O), is found in Wyoming and other parts of the world. There are various methods of recovering soda ash (sodium carbonate) from trona ore, for example, calcination. Generally the soda ash is recovered by digesting the calcined trona with water to dissolve out the soda ash content of the calcined ore. The resulting solution containing dissolved sodium carbonate is then treated to remove insolubles and the treated solution is then passed to evaporation units for crystallization of sodium carbonate monohydrate which is then separated and dried to produce the desired soda ash product. A portion of the waste liquors from the crystallization stage may be recycled to the digestion step. However, to limit the quantity of impurities in the system, a portion of the crystallization liquor is combined with the waste liquor from the muds separation stage and the mixture is discharged to an evaporation pond to reduce the alkaline waste liquor volume by solar evaporation, thereby preventing the pollution of the environment and utilizing radiant solar energy.
The general method of handling the large amounts of waste water produced in this process is to pass the waste liquors to a series of ponds where, by action of solar radiation, the volume of the liquid is decreased by evaporation of water therefrom. As the water is evaporated from these liquors, impurities present in the original liquor settle to the bottom as a thick sludge. The ponds are periodically cleaned to remove the sludge which has formed.
The major expense in use of such solar evaporation methods is the large surface area required to permit the solar evaporation to proceed from these ponds at a rate which allows continuous handling of the large volumes of water employed in typical commercial processes for manufacturing soda ash by the above-described method.
Accordingly, an increase in the rate of evaporation of these liquors would greatly assist in the processing of these waste liquors by the solar evaporation method, decreasing the surface area required for handling of the liquors produced in the trona process and thereby greatly decreasing the expense associated with the size and operation of these ponds.
While the prior art has employed soluble dyes in brines (e.g., sodium chloride and magnesium chloride brines) to increase rates of solar evaporation, no attempt has been made to define those dyes which would be suitable for use in the alkaline waste liquors produced in a trona process. See, for example, U.S. Pat. No. 2,383,763 and 3,099,630; Kane, G. P. et. al., "Acceleration of Solar Evaporation by Dyes"; Trans. Indian Inst. Chem. Engr., Vol. 3, 105-8 (1949-50); M. R. Bloch, et. al., "Solar Evaporation of Salt Brines"; Ind. & Engr. Chem., Vol. 43, 1544-1553 (1951); and Riva, B., Annali di Chimica (Rome), Vol. 50, 1391-1400 (1960).